The IRS website says there are several ways you might find out you were a victim of identity theft. Chances are high that if you were, you will discover it soon after filing your taxes. For example, the IRS may send you a letter saying you have filed more than one return, or that someone has filed a return using your information.
You may also find out by learning that you have a "balance due" or a "refund offset." Even worse, you may discover that the IRS is trying to collect from you for a year you didn't file taxes, meaning the IRS thinks you owe them money. (If you are using a tax preparation service, they may be the ones to tell you, but you will most likely get a letter soon after that from the IRS telling you the same thing.)
There are actions you should take to report a stolen identity. The IRS has created a unit for investigating identity theft and published a reporting form for the public to file.
Form 14039: When and How to File
Identity theft has become common enough that the IRS has a specific form to file—the IRS Identity Theft Affidavit (Form 14039)—for notifying them that you believe you are a victim of identity theft. The form may be filled out online, then printed and mailed or faxed. Information for submitting the form is on page two of Form 14039 (depending on how it was printed it, this may be on the back of the form). There is also a Spanish version available (Form 14039SP).
When sending in Form 14039, you will need to send a photocopy of an acceptable form of identification: a valid passport, your driver's license (issued in one of the US states), your social security card, or another valid form of U.S. Federal or State government-issued identification.
If you haven't been a victim of identity theft yet, but think that you might have to worry about it (for example if you had your Social Security card in your purse or wallet and it was recently stolen) you can still file Form 14039 to warn the IRS to flag your account for possible problems in the future. This doesn't guarantee that you won't still have problems, but it will give you a leg-up in dealing with the situation if it does arise.
Some of the reasons listed for suspecting you may become a victim of identity theft include a lost or stolen wallet or questionable information on your credit report. In most cases, it is a good idea to have a police report documenting a stolen wallet, purse, or phone.
More Identity Theft Protection Resources
The IRS also advises that if you believe you may become a victim of identity theft, you should call the Identity Protection Specialized Unit (IPSU), toll-free at 1-800-908-4490. The IPSU is available from 7 A.M to 7 P.M. in your local time zone. (Hawaii and Alaska follow the Pacific time zone for this.)
If you believe your identity has been stolen, you should also report your stolen identity to the Federal Trade Commission and the local police.
Some other actions you can take to protect your identity are to not carry documents with your social security number or taxpayer identification number, shred your official documents, protect your financial information, and use an identity theft protection service. Check your credit once a year, and make sure you regularly update all of your passwords. Finally, you should not give personal information over the phone or electronically unless you know who you're speaking with, and it is necessary to do so.